Hitherto, there has been known a structure of a knitting machine that has tension sensors confronting to the knitting yarn feeding pathway so that becomes controllable of knitting yarns to be fed to knitting needles at a desired tension (for example, see Patent Citation 1). By suppressing fluctuation of the knitting yarn tension when a fabric is being knitted, sizes of stitch loops are able to be kept uniform. In a knitting fabric, hand value is lost unless sizes of stitch loops achieve a suitable relation to the thickness of the knitting yarn used. In a flatbed knitting machine, when general knitting yarns are used, the thickness of the knitting yarn is chosen to nearly correspond to the gauge that indicates the number of knitting needles per 25.4 mm (1 inch). In accordance with the knitting yarn thickness, the knitting yarn tension is chosen, too, in such a manner that the stitch loop size that can provide the hand value as a suitable fabric is achieved.
Of the knitting fabrics knitted with the knitting machine, for portions requiring large retractility properties, for example, for the wearing openings of socks and gloves, etc., elastic yarns with particularly large extension coefficient as compared to general knitting yarns are used. Elastic yarns are also called rubber yarns, etc., and are made from polyurethane fibers, polyether/ester based fibers, and other fibers with large elasticity and retractility properties. In the elastic yarns, other fibers are used in combination, together with fiber materials with large retractility properties. For example, in the structures called covered yarns, core span yarns, etc., the outside of core fiber with large retractility properties, is covered with other fibers. 1
The elastic yarns are sometimes used not as ground yarns which construct knitting fabric itself but as inserted yarns which are inserted in a knitting fabric. The elastic yarns which are used as inserted yarns are used for knitting in the elongated state with comparatively large tension applied and in the fabric after knitting, tension is released and the elastic yarns shrink. To use elastic yarns, and to controll yarn tension and feed length, enables to knit a fabric in a finished state nearly close to the hand value corresponding to the gauge larger than the gauge of the knitting machine used (for example, see Patent Citation 2).    Patent Citation 1: U.S. Pat. No. 3,858,416 Specifications    Patent Citation 2: International Publication WO04/094712 pamphlet